libmbim-glib: return NULL on method returning string

No big deal anyway due to the g_assert_not_reached(), so just to make
compilers happy.
diff --git a/src/libmbim-glib/mbim-cid.c b/src/libmbim-glib/mbim-cid.c
index 69bb84c..8b47c5e 100644
--- a/src/libmbim-glib/mbim-cid.c
+++ b/src/libmbim-glib/mbim-cid.c
@@ -338,6 +338,6 @@
         return mbim_cid_qmi_get_string (cid);
     default:
         g_assert_not_reached ();
-        return FALSE;
+        return NULL;
     }
 }